1. Field of the Invention
The present invention relates to a method and apparatus for automatically correcting aberrations in a charged-particle beam and also to a method of controlling an aberration corrector for a charged-particle beam.
2. Description of Related Art
A scanning electron microscope (SEM) is an example of a surface imaging apparatus using charged particles. A surface imaging apparatus is described, taking a scanning electron microscope as an example. FIG. 7 is a diagram showing the structure of a prior art scanning electron microscope equipped with an aberration corrector. The microscope has an emitter 1 emitting an electron beam 2. The trajectory of the beam 2 incident on the aberration corrector 6 is controlled by a lens 3 acting on the beam 2. The beam 2 transmitted through the corrector 6 is focused onto the surface of a sample 5 by an objective lens 4. At this time, the aberration corrector 6 corrects the aberration of the total lens system.
The sample surface is scanned with the electron beam 2. Secondary electrons 7 are emitted from the sample surface in synchronism with the scanning and are detected by a secondary electron detector 8. Information about the sample surface based on the detected secondary electrons 7 is displayed as a visible image on a CRT 12 in synchronism with the scan signal. Normally, the efficiency of the secondary electron detector 8 which detects secondary electrons 7 is low. Therefore, noise is removed by an image accumulator 9. The image from which the noise has been removed in this way is displayed on the CRT 12. An aberration corrector equipped with multipole elements is known as an example of such aberration corrector 6 (see, for example, Zach et al. “Aberration correction in a low voltage SEM by a multipole corrector” (Nuclear Instruments and Methods in Physics Research, A 363 (1995), 316-325).
The technique disclosed in the cited reference is described below as an example. FIG. 8 is a diagram showing the configuration of an aberration corrector and the trajectory of an electron beam passing through the corrector. The aberration corrector, indicated by numeral 6, has four stages of multipole elements, which are shown, respectively, as first stage of multipole elements 6a, second stage of multipole elements 6b, third stage of multipole elements 6c, and fourth stage of multipole elements 6d. Normally, each stage of multipole elements has eight or more pole elements. The electron beam 2 incident on the aberration corrector 6 undergoes a focusing action and a diverging action simultaneously by the lens action of the first stage of multipole elements 6a. As shown, it is assumed that the electron beam 2 undergoes the diverging action in an X trajectory 6e and that the beam 2 undergoes the focusing action in a Y trajectory 6f. 
The lens strength of the first stage of multipole elements 6a is so adjusted that the Y trajectory 6f passes through the center of a lens field produced by the second stage of multipole elements 6b and is focused in the Y-direction. The lens strength of the second stage of multipole elements 6b is so adjusted that the X trajectory 6e passes through the center of a lens field produced by the third stage of multipole elements 6c and is focused in the X-direction. At this time, in the second stage of multipole elements 6b, the Y trajectory 6f passes through the center of the lens field produced by the second stage of multipole elements 6b and is focused in the Y-direction and so only the X trajectory 6e undergoes the lens action of the second stage of multipole elements 6b. Similarly, in the third stage of multipole elements 6c, the trajectory 6e passes through the center of the lens field produced by the third stage of multipole elements 6c and is focused in the X-direction. Therefore, only the Y trajectory 6f undergoes the lens action of the third stage of multipole elements 6c. In this way, the requirement is that the Y trajectory 6f of the electron beam 2 is focused on and passes through the center of the lens field produced by the second stage of multipole elements 6b and the X trajectory 6e of the beam 2 is focused on and passes through the center of the lens field produced by the third stage of multipole elements 6c. 
In the prior art, these corrections (alignments) of the trajectories of the electron beam have been made by adjusting the lens strengths of the plural stages of multipole elements 6a-6d while viewing the amount of movement of the image produced when the lens strengths of the stages of multipole elements 6a-6d are wobbled.
In particular, there are mechanical deviations among the center axes of the various stages of multipole elements 6a-6d. It is difficult to correct these mechanical deviations of the center axes among the stages of multipole elements 6a-6d by adjusting the mechanical positions of the stages of multipole elements 6a-6d. Accordingly, the conventional method is to adjust the lens field strength electrically produced by the stages of multipole elements 6a-6d to the appropriate electron beam pass. Consequently, substantial hindrance is prevented even if there are mechanical positional deviations among the stages of multipole elements 6a-6d. 
The lens fields produced by the stages of multipole elements 6a-6d contain dipole and quadrupole fields. When the dipole field produced by the multipole elements to be adjusted is adjusted, the quadrupole field produced by the multipole elements located behind the multipole elements to be adjusted is made to wobble. The dipole field produced by the multipole elements to be adjusted is adjusted such that the image taken during the wobbling does not move. In this way, the trajectory of the electron beam is aligned.
As described previously, it is necessary that the Y trajectory 6f of the beam 2 is focused on and passes through the center of the lens field produced by the second stage of multipole elements 6b and the X trajectory 6e of the beam 2 is focused on and passes through the center of the lens field produced by the third stage of multipole elements 6c. To achieve this requirement, the quadrupole fields produced by the stages of multipole elements 6a-6d need to be adjusted. During this adjustment, the dipole field produced by the multipole elements located next to the multipole elements to be adjusted is made to wobble. The quadrupole field produced by the multipole elements to be adjusted is adjusted such that the image obtained during the wobbling of the dipole field does not shift. In this way, the trajectory of the electron beam pass is aligned.
The procedure of adjustment of the dipole field produced by the aforementioned multipole elements is linked to the procedure for adjustment of the quadrupole fields produced by the other multipole elements such that the electron beam trajectory adjusted by one of these two kinds of fields is affected by adjustment of the other kind of field. Therefore, the operator has spent a long time in adjusting the lens field (multipole field) of each multipole element while checking the motion and quality of the image during wobbling, relying on his experience and intuition.
As described previously, in the aberration corrector composed of multiple stages of multipole elements, there are deviations among the mechanical center axes of the stages of multipole elements. By adjusting the multipole fields (dipole and quadrupole fields) produced by the stages of multipole elements by the aforementioned method (i.e., given multipole field produced by the multipole elements located behind the multipole elements to be adjusted is made to wobble), the operator empirically aligns the trajectory of the electron beam such that the beam is focused and passes through the center of the corresponding stages of multipole elements within the aberration corrector.
To achieve this, the operator manually adjusts and corrects the lens strengths of the stages of multipole elements. These procedural operations are complexly linked. There is the problem that the operator spends a lot of time in making the manual adjustment according to the movement and quality of the image while relying on his experience and intuition.